Image-reproducing device



Nov. 3, 1953 R. s. PETERMAN IMAGE-REPRODUCING DEVICE Filed Nov. 23, 1951INVENTOR. RUSSEL S. 'PETERMAN HIS ATTORNEY.

Patented Nov. 3, 1953 IMAGE-REPRODUCING DEVICE Russel S. Peterman,

Des Plaines, Ill., assignor to The Rauland Corporation, a corporation ofIllinois Application November 23, 1951, Serial No. 257,692 Claims. (01.313-82) invention relates to image-reproducing devices and moreparticularly to cathode-ray tubes for use as picture-reproducing devicesin television receivers and the like.

One of the problems often encountered in the production of televisionreceivers stems from the susceptibility of commonly employed cathode-raytube constructions to exhibit extraneous cold emission and sparkdischarge in the electrode system. Whenever two closely spacedelectrodes are maintained at large potential differences, theseundesirable phenomena are apt to be encountered. For example, mostelectron guns comprise a cathode, a control grid, a so-called secondgrid which is maintained at a low constant positive potential withrespect to the oathode, and a tubular anode closely spaced from thesecond grid and maintained at a much higher positive potential. Due tothe large potential difference and the close spacing between the tubularanode and the second grid, cold emission and spark discharge may lead toa condition commonly referred to as popover which is manifested by amomentary loss of brightness or a tearing out of minor portions of thereproduced image. The problem is further aggravated in the case ofelectrostatically focused cathode-ray tubes employing unipotentialfocusing lens systems, particularly in the case of such systems whereinthe focusing electrode is maintained at or near cathode potential, dueto the extremely high voltage gradients between the focusing electrodeand the other two elements of the focusing lens.

It is an important object of the present invention to provide a new andimproved image-reproducing device in which cold emission and sparkdischarge in the electrode system are effectively inhibited.

It is a further object of the invention to provide a new and improvedimage-reproducing device in which the objective of reduced cold emissionand spark discharge is accomplished by means of a simple and inexpensivemodification of the tube structure.

In accordance with the present invention, a new and improved cathode-raytube comprises an electrode system for projecting an electron beam. Theelectrode system includes a cathode and a plurality of tubularelectrodes supported within an evacuated envelope, and a conductivecoating on the inner wall of the envelope. An additional conductivecoating on the inner wall of the envelope, substantially shielded fromthe path of the elect on b am. at least partially lencompassing thetubular electrodes and electriconnection with the accompanying drawing,in

which the single figure is a fragmentary side elevation, partly incross-section and partly cut away, of an image-reproducing deviceconstructed in accordance with the present invention.

The image-reproducing device of the figure comprises a fluorescentscreen l0 afiixed to the glass target portion ll of a cathode-ray tubeenvelope which also comprises a glass neck portion I 2 enclosing anelectron gun and an electrostatic focusing system. The electron guncomprises a cathode I 3, a control electrode l4, and first and secondtubular accelerating electrodes l5 and I6 respectively. A diaphragm I!having a central aperture I8 is disposed across the outlet end of secondaccelerating electrode l6, and aperture I8 is symmetrically centeredwith respect to the tube axis A-A perpendicular to the center of thefluorescent screen I0. Second accelerating electrode 5 is laterallyoffset from first accelerating electrode F5 to provide a steadytransverse electrostatic-deflection field component in the regionbetween these two electrodes,

and the entire electron gun structure is tilted with respect to the tubeaxis A-A by an angle 0.

An electrostatic focusing system of the unipotential lens type isdisposed between the electron gun and the fluorescent screen. Thefocusing system comprises the outlet end of second acceleratingelectrode l6 including diaphragm IT, a lens electrode I9, and anadditional electrode 20 which are all coaxially mounted with respect tothe tube axis AA. A centrally apertured conductive disc 2| is disposedin the neck portion of the envelope between the focusing system andtarget portion H. A conductive coating 22, of

:colloidal graphite such as aquadag or the like.

extends from the direction of target portion ll into the neck portion ofthe envelope, and conductive disc 2| is maintained at a common potentialwith conductive coating 22 by means of metal contact springs 23.

For convenience, electrodes 14, I 5, I6, I 9 and 20 may be termed gridsand may be designated by number starting with control electrode I4 asthe first grid and progressing in the direction of beam travel toadditional electrode 20 which is the fifth grid. All five grids aresuported in predetermined mutually spaced relation by means of a pair ofglass pillars 24, of which only one is shown, in a manner which will beapparent to those skilled in the art. Separate leads for grids I, 2 and4 extend through the base 25 of the tube, as do the supply leads for thecathode I3 and its associated heater element (not shown). Lead 26 fromgrid 4 through the base of the tube may be provided with an insulatingglass bead (not shown) to inhibit spark discharge to electrode I6.Conductive disc H is mechanically supported from and electricallyconnected to grids 3 and 5 by means of metal connecting strips 21.Operating potential for the conductive coating 22, and therefore for thethird and fifth grids, may be supplied by means of a conventionalcontact button if the envelope is of the all glass type, or directly tothe metal cone member if the tube is of the glass-metal variety.

An external permanent magnet 28, supported in a spring clamp 29 whichfits snugly around the neck of the tube and is movable both axially androtationally, is provided to develop a magnetic field within the tube toprovide separation of the negative ions from the electron beam.

The tube is evacuated, sealed and based in accordance with well knownprocedures which require no further explanation, and suitable getters 3|are supported from the surface of conductive disc 2I facing fluorescentscreen III to absorb residual gases after evacuation.

In operation, a mixed beam of electrons and negative ions originating atcathode I3 is projected through the aperture in second grid I5. When themixed beam emerges from grid 2, it encounters an electrostatic fieldhaving a transverse component due to the lateral offset of grid 3 withrespect to grid 2. Consequently, electrons and ions are both deflectedto the left in the view of the figure. The magnetic field imposed bybeam-bender magnet 28 serves to defiect the electrons to the right asviewed in the figure without substantially affecting the path taken bythe negativ ions. Thus, when beam-bender magnet 28 is accuratelyadjusted, the beam of electrons is projected centrally through apertureI8 of diaphragm IT in a direction along the tube axis AA, while thenegative ions are intercepted by the metallic portions of grid 3 anddiaphragm II. The ion-trap mechanism is disclosed and claimed in thecopending application of Willis E. Phillips et al., Serial No. 156,746,filed April 19, 1950, for Electron Gun for Cathode- Ray Tubes, nowPatent No. 2,596,508, issued May 13, 1952, and assigned to the presentassignee.

The axially directed electron beam is subjected to the focusing actionof the electrostatic fields produced by diaphragm II, lens electrode I9and the fifth grid 20 which together constitute a unipotentialelectrostatic focusing lens system. The general construction andoperation of lenses of this type are well understood by those skilled inthe art as indicated by an article entitled Measured properties ofstrong unipotential electron lenses by G. Liebmann, Proceedings of thePhysical Society, Section B, volume 62, part 4, pages 213-228 (April 1,1949) The required operating potential difference between the lenselectrode (grid 4) and the other electrodes of the focusing system(grids 3 and 5) is determined by the dimensions Di and the spacingbetween the electrodes constituting the unipotential lens. Although therelationships are not necessarily linear, the required focusingpotential difference varies directly with the length and inversely withthe diameter of grid 4, and inversely with the separation between grid 4and grids 3 and 5. Certain limitations on these parameters are imposedby practical considerations; if the diameter of grid 4 is made toosmall, excessive spherical aberration is encountered, and if theseparation between grids 3 and 4 is made too great, the deflectinginfluence exerted by the asymmetrical electrostatic field establishedbetween lead wire 26 and grid 3 becomes objectionable. The focusingsystem is preferably constructed and arranged to obtain focusing withgrid 4 operated at or near cathode potential, in order to avoid thenecessity of providing a source of operating potential intermediate theB-supply voltage and the final anode voltage.

In order to obtain satisfactory focusing with grid 4 operated at apotential between cathode potential and the B-supply voltage, it isnecessary to maintain rather stringent manufacturing tolerances withrespect to the dimensions and spacings of the several electrodesconstituting the focusing system. In addition, when grid 4 is operatedat a potential substantially equal to that of the cathode, extremelyhigh voltage gradients are produced between grid 4 and grids 3 and 5. Inorder to suppress undesirable corona effects and field emission, grids 3and 5 are each provided with corona rings 32 and 33 in the form ofrolled flanges of stainless steel or the like which are welded orotherwise secured to the respective electrodes, and grid 4 isconstructed by rolling the two ends of a metal cylinder 34 over the edgeof a large aperture in a metal disc 35.

Corona rings 32 and 33 also perform an additional function infacilitating the maintenance of the required close manufacturingtolerances by mechanically reinforcing the circular flanges to whichthey are attached against warping or bending during the assembly of theelectrode system. The electrodes are assembled by means of accuratelyconstructed jigs and are all rigidly supported by means of opposedcommon glass pillars 24, the gun assembly being properly oriented in thetube neck by means of other jigs in the usual manner. It has been foundthat these precautions suflice to insure satisfactory operation of thecompleted structure, any small deviations in dimensions and spacingsbeing readily compensated by adjustment of the ion-trap magnet 28.

For best results, it has been found that the apertures in grids I, 2, 3and 5 should be in marginally overlapping alignment in a directionparallel to the tube axis AA. In other words, all of these aperturesshould intercept an imaginary straight line parallel to reference axisAA, and the apertures in grids I and 2 should intercept that lineasymmetrically. Fulfillment of this condition is dependent upon theangle 0 by which the entire electron gun is tilted with respect to thetube axis, and also upon the length of the electron gun from the cathodeto aperture I8 in diaphragm I1. If the angle 0 and/or the length of thegun is increased to such an extent that the apertures in grids I, 2, 3and 5 are no longer in marginally overlapping alignment in a directionparallel to the tube axis, increased multiplicity of focus isencountered, and the performance of the focusing system is inferior. Onthe other hand, if the angle 0 is decreased so that the apertures are incomplete coaxial alignment, ion trappin may no longer be convenientlyaccomplished.

By employing a separate conductive disc 2! for establishing electricalcontact to conductive coating 22, and by terminating conductive coating22 at substantially the plane of the conductive disc 2 1, high potentialgradients and undesirable spark discharge between the low-potential lenselectrode i9 and the high-potential conductive coating 22 aresubstantially avoided. Moreover, even though this construction resultsin a space between grid 5 and conductive disc 2i in which the boundarypotentials are not definitely established, no observable distortion ordefocusing of the beam is encountered. The size of the aperture inconductive disc 2| is not critical but should be large with respect tothe apertures in grids 3 and 5.

Moreover, conductive disc 2! serves as an effective getter shield toavoid conductive deposits on glass support pillars 2 when the getter 3!is flashed during the processing of the tube. In this manner,excessively high potential gradients along the insulating pillars andpossible insulator breakdown are substantially avoided.

Due to the close spacings and the high potential differences betweengrids 2 and 3 and between grid :3 and grids 3 and 5, undesirable coldemission and spark discharge between these electrodes may beencountered. This undesirable situation may be avoided by observingevery precaution to insure that the glass envelope and all of theelectrodes are kept clean and free from grease or other organic matter,and by avoiding sharp edges on the closely spaced opposing portions ofthe several electrodes. However, it has been found in practice,particularly in the production of electrostatically focused picturetubes embodying unipotential focusing lens systems, that the requisitedegree of cleanliness cannot be efiectively maintained by the use ofcommercially feasible production methods.

In accordance with the present invention, undesirable cold emission andspark discharge in the electrode system are effectively inhibited byproviding a second conductive coating 49, separate from the conductivecoating 22 which is connected to grids 3 and 5 and maintained at highpotential, within the glass neck portion of the envelope in a positionat least partially encompassing the tubular electrodes 1 5 and it of theelectron gun. Conductive coating 39 is maintained at a constant lowpositive potential with respect to cathode i3, preferably by means ofcontact springs 4! connected to the second grid l 5. It has been foundthat a cathode-ray tube constructed in this manner may be operated withfinal anode voltages considerably in excess of those required in actualuse of the tube in a television receiver or the like withoutencountering spark discharge or popover in the electrode system.

The technical reasons underlying the success of the construction of thepresent invention are not fully understood. One theory which seems inagreement with numerous observations is predicated on the hypothesisthat cold emission and spark discharge are only encountered at normaloperating voltages in the presence of extraneous gas molecules. In aconventional image-reproducing device not provided with the conductivecoating 19 of the present invention, primary or secondary electronbombardment of the glass neck portion of the envelope is thought toresult in the liberation of occluded gases from grease,

' vice, and Robert W.

dirt, or other extraneous organic matter which may be present. These gasmolecules then become ionized and are caused to bombard the gunelectrodes, leading to cold emission which ultimately results in sparkdischarge. This theory is supported by the observation that troublesomespark discharge generally takes place along the glass wall of theenvelope in the first instance and only later across the shorterintervening space between the gun electrodes. By providing internalconductive coating 48, and by maintaining that coating at a low constantpositive potential by means of contact springs 41 connected to secondgrid l5, gas liberation is avoided for two reasons. In the first place,coating 39 acts as a physical barrier between the offending primaryand/0r secondary electrons and the glass wall of the envelope, thusrendering extraneous grease or dirt on the glass wall inaccessible. Inthe second place, such stray electrons as reach the wall of the tube arecollected by coating 40 and conducted oil through the external circuitassociated with the second grid.

While it is preferred to employ internal contact springs 4| to connectcoating 49 to second grid [5, in order to avoid the necessity for anadditional external lead from the envelope, the potential at whichconductive coating 40 is maintained is not critical, and it is withinthe scope or" the invention to provide other means independent of thegun electrodes for connecting coating 40 to an associated constantpotential source.

Coating 49 may be formed of silver paint or any other conductivematerial which is amenable to application in the form of an internalcoating. The length of coating 40 is not critical, although it isessential that the tubular gun electrodes be at least partiallyencompassed by the coating, and that coating 46 be maintained physicallyand electrically separate from the final anode coating 22.

Thus the present invention provides a new and improved image-reproducingdevice in which cold emission and spark discharge are effectivelyinhibited by the simple expedient of providing a conductive coating onthe inner wall of the glass neck, physically and electrically separatefrom the final anode coating and at least partially encompassing thetubular gun electrodes. It has been found that image-reproducing devicesconstructed in accordance with the present invention, whether of themagnetically focused or the electrostatically focused type, exhibit agreatly re-v duced tendency towards undesirable popover, even atoperating voltages greatly in excess of those normally employed.

Certain features of the construction illustrated and described in thepresent application are disclosed and claimed in the copendingapplications of Constantin S. Szegho, Serial No. 229,013, filed May 31,1951, for Image-Reproducing Device, Jerome J. OCallaghan, Serial No.235,045, filed July 3, 1951, now Patent No. 2,627,043, issued January27, 1953, for Image-Reproducing De- Shawfrank, Serial No. 234,920, filedJuly 3, 1951, now Patent No. 2,627,049, issued January 27, 1953, forCathode- Ray Tube Electrode, all of which are assigned to the presentassignee.

While a particular embodiment of the present invention has been shownand described, it is apparent that various changes and modifications maybe made, and it is therefore contemplated in the appended claims tocover all such changes and modifications as fall within the true spiritand scope of the invention.

I claim:

1. A cathode-ray tube comprising: an evacuated envelope; an electrodesystem within said envelope for projecting .an electron beam, saidelectrode system including a cathode, a plurality of tubular electrodes,and a conductive coating on the inner wall of said envelope; and asecond conductive coating on the inner wall of said envelope, separatefrom said first-mentioned coating, substantially shielded from the pathof said electron beam, and electrically connected to one of said tubularelectrodes, for inhibiting cold emission and spark discharge in saidelectrode system.

2. A cathode-ray tube comprising: an evacuated envelope; a fluorescentscreen supported within said envelope; an electrode system within saidenvelope for projecting an electron beam, said electrode systemincluding a cathode, a plurality of tubular electrodes, and a conductivecoating on the inner wall of said envelope between said tubularelectrodes and said fluorescent screen; means for electricallyconnecting said conductive coating to one of said tubular electrodes;and another conductive coating on the inner wall of said envelope,separate from said first-mentioned coating, substantially shielded fromthe path of said electron beam, and electrically connected to another ofsaid tubular electrodes, for inhibiting cold emission and sparkdischarge in said electrode system.

3. A cathode-ray tube comprising: an evacuated envelope; a fluorescentscreen supported within said envelope; an electrode system within saidenvelope for projecting an electron beam, said electrode systemincluding a cathode, a plurality of tubular electrodes, and a conductivecoating on the inner wall of said envelope between said tubularelectrodes and said fluorescent screen; means including a plurality ofcontact springs for electrically connecting said conductive coating toone of said tubular electrodes; another conductive coating on the innerwall of said envelope, separate from said first mentioned coating,substantially shielded from the path of said electron beam, and at leastpartially encompassing said tubular electrodes, for inhibiting coldemission and spark discharge in said electrode system; and additionalcontact springs for electrically connecting said other conductivecoating to another of said tubular electrodes.

4. A cathode-ray tube comprising: an evacuated envelope; a fluorescentscreen supported within said envelope; an electrode system within saidenvelope for projecting an electron beam, said electrode systemincluding a cathode, first and second grids, a tubular electrode, and aconductive coating on the inner wall of said envelope between saidtubular electrode and said fluorescent screen; means including aplurality of contact springs for electrically connecting said conductivecoating to said tubular electrode; another conductive coating on theinner wall of said envelope, separate from said first-mentioned coating,substantially shielded from the path oi! said electron beam, and atleast partially encompassing said second grid and said tubularelectrode, for inhibiting cold emission and spark discharge in saidelectrode system; and additional contact springs for electricallyconnecting said other conductive coating to said second grid.

5. A cathode-ray tube comprising: an evacuated envelope having a glassneck portion; a fluorescent screen supported within said envelope; anelectrode system for projecting an electron beam, said electrode system,including a cathode, first and second grids, and a tubular electrode allsupported within said glass neck portion, and a conductive coating onthe inner wall of said envelope electrically connected to said tubularelectrode, for projecting an electron beam toward said fluorescentscreen; and another conductive coating, on the inner wall of said neckportion, separate from said first-mentioned coating, substantiallyshielded from the path of the electron beam, and electrically connectedto said second grid, for inhibiting cold emission and spark discharge insaid electrode system.

RUSSEL S. PE'IERMAN.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,058,914 Rudenberg Oct. 27, 1936 2,070,319 Rudenberg Feb. 9,1937 2,170,251 Schlesinger Aug. 22, 1939 2,210,127 Rogowski Aug. 6, 19402,213,175 Iams et al Aug. 27, 1940 2,250,927 Davisson July 29, 19412,264,274 Broadway Dec. 2, 1941 2,275,864 Record Mar. 10, 1942 2,363,359Ramo Nov. 21, 1944 2,409,514 Pratt Oct. 15, 1946 2,414,881 Law Jan. 28,1947 2,452,919 Gabor Nov. 2, 1948 2,454,345 Rudenberg Nov. 23, 19482,496,127 Kelar Jan. 31, 1950 2,555,850 Glyptis June 5, 1951 2,562,242Pohle July 31, 1951 2,567,893 Pohle Sept. 11, 1951 2,572,858 HarrisonOct. 30, 1951

